Electrochemical treatment based surface modification device

ABSTRACT

The invention provides an electrochemical treatment based surface modification device that comprises a solution tank, a cathode terminal, and an anode terminal. The solution tank is filled with an acidic solution which contains first valence metal ions. The first valence metal ions are partially reduced to second valence metal ions at the cathode terminal. The valence of the first valence metal ion is greater than that of the second valence metal ion. The anode terminal is provided an electrically conductive oxide, and the second valence metal ions move from the cathode terminal to the anode terminal to from a metal oxide. Wherein, the deposition and etching of the conductive oxide occur simultaneously on the surface of the anode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This application claims priority of No. 101134076 filed in Taiwan R.O.C.on Nov. 19, 2012 under 35 USC 119, the entire content of which is herebyincorporated by reference.

The invention relates to a surface modification device, particularly toan electrochemical treatment based surface modification device.

2. Related Art

Tin-doped indium oxide (ITO), because of its excellent visible lighttransmittances and electric conductivities, has been the most widelyused transparent conducting oxide (TCO) in optoelectronic applications.ITO however suffers from the disadvantages of poor heat stability, highcost, and worsening electric conductivity with increasing temperatures.More importantly, indium is an Earth-scarce element and thus lacks thelong term supply stability. Consequently, there have been extensive andintensive research efforts to develop more stable, heat-durable, andcost-effective alternatives to replace ITO. Fluorine-doped tin oxide(FTO) is one of the few promising candidates. Although FTO is cheaperthan ITO, FTO glass has to be thicker than ITO glass because oftechnical difficulties involved in manufacturing processes. However,thicker glass will present longer light paths so that lighttransmittances in the FTO glass will be lower. The applications of FTOglass in optoelectronics are thus limited by the relatively lowervisible light transmittances.

Industry circle tries to improve the problem of lower visible lighttransmittances by usinganti-reflection films (e.g., TiO₂, SiO₂, andPVA). Although anti-reflection films can increase visible lighttransmittances, they also decrease the electric conductivity at the sametime.

SUMMARY OF THE INVENTION

An objective of the present invention is to provide a device forincreasing light transmittance of conductive oxide films.

An objective of the present invention is to provide a device for costsaving.

An objective of the present invention is to provide a device fordecreasing surface roughness of conductive oxide films.

The invention provides an electrochemical treatment based surfacemodification device that comprises a solution tank, a cathode terminal,and an anode terminal. The solution tank is filled with an acidicsolution which contains first valence metal ions. The first valencemetal ions are partially reduced to second valence metal ions at thecathode terminal. The valence of the first valence metal ion, is greaterthan that of the second valence metal ion. The anode terminal isprovided an electrically conductive oxide, and the second valence metalions move from the cathode terminal to the anode terminal to form ametal oxide. Wherein, the deposition and etching of the conductive oxideoccur simultaneously on the surface of the anode terminal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram illustrating an electrochemicaltreatment based surface modification device according to one embodimentof the invention.

FIG. 2 shows the cross-sectional and top view SEM images of the FTOlayer before and after the electrochemical treatment.

FIG. 3A shows an evolution of surface morphology, layer thickness, andsheet resistance of the FTO layer.

FIG. 3B shows transmittance spectra of untreated and treated FTO samplesat five different treatment temperatures. Insets are corresponding topview SEM images.

FIG. 4 shows XRD patterns of untreated and treated FTO samples. Inset isa local enlargement of the (110) diffraction peak.

FIG. 5 shows XRD pattern of corresponding deposits collected at cathode.

FIG. 6 shows XPS spectrum of FTO sample treated at 5V and 60° C. for 30min.

DETAILED DESCRIPTION OF THE INVENTION

Please refer to FIG. 1. FIG. 1 shows a schematic diagram illustrating anelectrochemical treatment based surface modification device according toone embodiment of the invention. Surface modification device 10comprises a solution tank 10 a, a cathode terminal C, and an anodeterminal A.

Solution tank 10 a is filled with an acidic solution L. Acidic solutionL has tetravalent tin ions (Sn⁴⁺). In the present embodiment, acidicsolution L can be implemented by nitric acid. The source of Sn⁴⁺ can beimplemented from stannic chloride (SnCl₄). There has 5V between cathodeterminal C and anode terminal A.

In the present embodiment, cathode terminal C can be implemented byplatinum (Pt). Anode terminal A provides a conductive oxide J, theconductive oxide J is implemented by fluorine-doped tin oxide (FTO).

Wherein, cathode terminal C has following reactions (1)˜(4):

Sn⁴⁺+2e ⁻→Sn²⁺  (1)

Sn²⁺+2e ⁻→Sn   (2)

Sn⁴⁺+4e ⁻→Sn   (3)

2H³⁰ +2e ⁻→H₂   (4)

It should be noted that, as shown in reaction (1), Sn⁴⁺ are reduced todivalent tin ions (Sn²⁺) through partial reduction. We can know that thevalence of Sn⁴⁺ is greater than valence of Sn²⁺. But the invention isnot limited to the tin ion. It can be implemented by other metal ions.

Besides, as shown in reactions (2) and (3), Sn⁴⁺ and Sn²⁺ can also bereduced to Sn.

Moreover, anode terminal A has following reactions (5)˜(8):

2H₂O→O₂+4H⁺+4e ⁻  (5)

2Sn²⁺+O₂+6H₂O→2Sn(OH)₄+4H⁺  (6)

Sn(OH)₄+Sn(OH)₄→2SnO₂+4H₂O   (7)

SnO₂+4H⁺→Sn⁴⁺+2H₂O   (8)

The detailed description for anode terminal A is as follows. Firstly,H₂O is oxidized to form O₂ and H⁺ (as shown in reaction (5)). H⁺ will bereduced to H₂ at cathode terminal C (as shown in reaction (4)). The Sn²⁺is supplied through mass transfer from the partial reduction of Sn⁴⁺atthe cathode terminal C. Then, Sn²⁺ reacts with H₂O and O₂ to first formSn(OH)₄ (as shown in Reaction (6)), that further goes through acondensation reaction to form SnO₂ on the anode terminal A (FTO) surface(as shown in Reaction (7)).

It should be noted that, as shown in reaction (8), SnO₂ of FTO surfacewill react with H⁺, wherein, H⁺ are generated from reactions (5) or (6).On the other hand, the H⁺ produced in the proximity of the anodeterminal A surface from reactions (5) and (6) performed the etching ofSnO₂. Consequently, deposition and etching of SnO₂ occurredsimultaneously at the anode terminal A (FTO), the balance of whichleading to the flattening of the FTO surface. Therefore, electrochemicalreaction time in device 10 can be adjusted according to user's demand.

Please refer to FIG. 2. FIG. 2 shows the cross-sectional and top viewSEM images of the FTO layer before and after the electrochemicaltreatment. Here, untreated FTO glass (sheet resistance 6-8 Ω/sq, 2 mm×20mm×20 mm) is used as the object for the flattening process. Theelectrochemical treatment is operated at 5V and 60° C. for 30 minutes.The cathode terminal C and anode terminal A are kept 25 mm apart and theelectrolyte is stirred at ambient condition.

In the present embodiment, under processing time 30 minutes andenvironment temperature 60° C., FTO surface will be the smoothest. It isbecause that nitrate ions (NO³⁻) in the cathode terminal C are reducedto NO ions, but NO ions will oxidize Sn²⁺ to Sn⁴⁺ and impede the amountof Sn²⁺ to deposit on anode terminal A when environment temperature isless than 60° C. Conversely, when environment temperature is greaterthan 60° C., cathode terminal C will generate relatively few NO⁺, sothat the deposition of Sn²⁺ on anode terminal A will increase and makeFTO surface rougher. Therefore, the present embodiment is based on 30minutes and environment temperature 60° C. of the electrochemicaltreatment.

By (a) and (b) in FIG. 2, evidently, the originally much rougher surfaceis replaced by a dense, smooth surface. From the top view SEM images,granular structure is evident for the untreated sample, whereas almostno structural features can be observed from the treated sample,indicating again the much improved surface flatness of the treatedsample. As shown in (A) and (B) of FIG. 2, the layer thickness isincreased from 640 nm to 755 nm by around 115 nm over a treatment periodof 30 min at 5V, and the surface roughness is decreased from 15 to 5 nmas determined with an AFM. Presumably, thicker films exhibit lower lighttransmittances. The visible light transmittances of the treated sample,as discussed in a later section, however are increased by 6% (from 79 to85% at 550 nm). Evidently, the gain in light transmittances through thesuppression of light scattering at the FTO-air interface for the muchflattened treated sample over-compensates the loss in transmittancesfrom the thickness increase.

Please also refer to FIG. 3A. FIG. 3A shows an evolution of surfacemorphology, layer thickness, and sheet resistance of the FTO layer.Wherein, left axis shows sheet resistance of FTO and right axis showsthickness.

After the electrochemical treatment, FTO acquires the flattest surface,while the sheet resistance is only slightly increased from 7.7 Ω to 14Ω. The thickness is increased from 640 nm to 755 nm under theenvironment temperature of 60° C. In the present embodiment, even thoughelectrochemical treatment is controlled under environment temperature of40° C., or 50° C., or 70° C., or 80° C., FTO surface is still rougherthan that obtained from the environment temperature of 60° C.

Please also refer to FIG. 3B. FIG. 3B shows transmittance spectra ofuntreated and treated FTO samples at five different treatmenttemperatures. Insets are corresponding top view SEM images. Lighttransmittance of untreated FTO is about 79%. It can be increased to atleast 85% after the electrochemical treatment. Therefore, the FTO filmthickness increases after the electrochemical treatment; the sheetresistance is slightly increased, and the light transmittance of the FTOincreases from 79% to 85%. Wherein, when the illumination wavelength is550 nm, the increment of light transmittance is about 6%. This showsthat although the thickness of the treated FTO is increased, FTO surfacebecomes relatively flat and avoids light scattering to increase lighttransmittances.

Then, please also refer to FIG. 4. FIG. 4 shows XRD patterns ofuntreated and treated FTO samples. Inset is a local enlargement of the(110) diffraction peak. Both patterns matched very well with that ofSnO₂ of the tetragonal phase (JCPDS 77-0447). No extra diffraction peakscan be identified from both patterns, indicating that SnO₂ was the solecrystalline product at the anode after the treatment. If one examinesthe (110) diffraction peaks closely as enlarged in the inset, there canbe observed left-shifts in 2θ of the F-doped samples from that of theSnO₂. The left-shift in 2θ was caused by the F-doping, and was morepronounced for the untreated FTO sample because of its higher dopingconcentrations. Here, substitution of one O²⁻ by two F⁻ is necessary tomaintain electroneutrality, and thus results in an increase in latticeparameters, giving left-shifts in 2θ.

Furthermore, inset is a local enlargement of the (110) diffraction peak.The fluoride ion doping can be proved by the inset of FIG. 4. Grain sizeof SnO₂ is enlarged when fluoride ions are doped into the crystalstructure of SnO₂. According to Bragg's law, 2θ will shift to smallerangles when the grain size of SnO₂ becomes larger. As shown in FIG. 4,the maximum 20 shift level is observed for the untreated FTO, next isthe treated FTO, and the lowermost peak is for the un-doped SnO₂. Fromthe foregoing, the material deposited by the electrochemical treatmentof the present invention is FTO crystals.

Please refer to FIG. 5. FIG. 5 shows XRD pattern of correspondingdeposits collected at cathode terminal C. FIG. 5 is to verify that thedeposited product on the surface of cathode terminal C during theelectrochemical treatment is Sn. FIG. 5 shows the XRD pattern of thecathode after electrochemical treatment at 5V and 60° C. for 30 min.Expectedly, the product obtained at the cathode is Sn (JCPDS 89-4898)from the reduction of Sn⁴⁺ and Sn²⁺.

Then, please refer to FIG. 6. FIG. 6 shows the XPS spectrum of the FTOsample treated at 5V and 60° C. for 30 min.

To further confirm the chemical composition of the deposit at the anode,the surface elemental composition of the treated sample prepared at 5Vand 60° C. for 30 minutes is determined with XPS. The newly depositedlayer is 165 nm in thickness, which is able to well shield the base FTOlayer from being sampled by the XPS measurement. FIG. 6 shows the XPS ofthe treated FTO sample, which displays an evident spin-orbit doublet at486.8 (3d_(5/2)) and 495.3 eV (3d₃₁₂) for the confirmation of theoxidation state of Sn⁴⁺ for the smooth dense layer, proving theformation of SnO₂ as the product.

While the present invention has been described by the way of examplesand in terms of preferred embodiments, it is to be understood that thepresent invention is not limited thereto. To the contrary, it isintended to cover various modifications. Therefore, the scope of theappended claims should be accorded the broadest interpretation so as toencompass all such modifications.

In conclusion, a novel, facile, one-step Sn⁴⁺-based anodic depositionprocess is developed, by which flattening and thus transmittanceenhancements of the FTO layer are achieved for commercial FTO glass. Theunique design of the indirect and in-situ supply of Sn²⁺ from thestarting Sn⁴⁺ through partial reductions for the anodic deposition ofSnO₂ and the use of HNO₃ for controlled SnO₂ deposition rates at theanode is critical for the slow and balanced SnO₂ deposition and etchingto create the significantly flattened dense film. Consequently,utilizing the present electrochemical treatment on FTO surfaces willmake FTO smoother and increase light transmittances. FTO can replace theITO as a transparent conductive material.

What is claimed is:
 1. An electrochemical treatment based surfacemodification device comprising: a solution tank filling with an acidicsolution, which is comprised first valence metal ions; a cathodeterminal, at which the first valence metal ions are reduced to a secondvalence metal ions through partial reduction; and an anode terminal atwhich a conductive oxide is provided and the second valence metal ionsmove from the cathode terminal to the anode terminal to form a metaloxide; wherein, the deposition and etching of the conductive oxide occursimultaneously on the surface of the anode terminal.
 2. The deviceaccording to claim 1, wherein the first valence metal ions or the secondvalence metal ions are reduced to a metal on the cathode terminal. 3.The device according to claim 1, wherein the first valence metal ionsare tetravalent metal ions and the second valence metal ions aredivalent metal ions.
 4. The device according to claim 3, whereinhydrogen ions and oxygen are formed from oxidation of water on the anodeterminal.
 5. The device according to claim 4, wherein the metalhydroxide is synthesized from the second valence metal ions and water.6. The device according to claim 5, wherein the metal hydroxide convertsto the metal oxide on the anode terminal through condensation reaction.7. The device according to claim 6, wherein the metal oxide on the anodeterminal is etched, and the tetravalent metal ions and fluorine ions arereleased from the anode terminal.
 8. The device according to claim 7,wherein metal Sn is formed from reduction of tetravalent metal ions oncathode terminal, and some parts of tetravalent metal ions are depositedwith fluorine ions on the anode terminal.
 9. The device according toclaim 8, wherein the metal oxide is fluorine-doped tin oxide (FTO). 10.The device according to claim 1, wherein the acidic solution is nitricacid.